The first aerogels, made in the the 1930's by S. S. Kistler (Nature 127: 741 (1931)) and disclosed in U.S. Pat. No. 2,249,767 were translucent pieces of porous silica glass which were prepared by formation of silica `hydrogels` that were initially exchanged with alcohol, then dried with little shrinkage. When alcohol was supercritically extracted from the wet gel under pressure and at high temperature, the aerogel produced had a density of about 0.05 g/cm.sup.3, or a porosity as high as 98 percent. However, Kistler's process was time consuming and laborious and subsequent advances in the art, have reduced the processing time and increased the quality of the aerogels produced.
The high porosity silica aerogels first made were scientific curiosities and were not used in practical applications. Recently, in the 1970's, the high porosity aerogels were used as part of detectors for charged particles in high-energy physics experiments. The aerogels have a thermal conductivity which is about 100 times less than conventional, non-porous silica glass. The high transparency of silica aerogels, combined with their excellent insulating properties, suggest that aerogels could serve as superinsulating window materials. Other practical applications for silica aerogels include use as insulation in refrigerators, boilers, or as passive solar collectors.
Sound transmission through aerogels is slower than through air and their acoustic impedance is intermediate between that of most sound transducers and air. This suggests that aerogels could be used to improve the efficiency of such transducers in applications such as micro-speakers and distance ranging. The unique properties of aerogels make them useful for a variety of applications which require transparency, low thermal conductivity and strength with very low weight.
An improvement over Kistler's method was described by Teichner et al., in U.S. Pat. No. 3,672,833, where a silicon alkoxide such as tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS), in an aliphatic alcohol solvent, was hydrolysed by one to five times the stoichiometric quantity of water, in a single mixing step. This single-step sol-gel process involves the hydrolysis reaction of silicon alkoxide compounds with water in either an acid, neutral, or basic medium, followed by the condensation reactions in which the hydrolysis products polymerize to form a gel. In this method, the wet gel already contains alcohol solvent as a result of the reactions, and therefore does not require the slow process of exchanging solvents before drying by supercritical extraction, as does Kistler's method. Also, the alcohol can be directly removed from the wet gel at high temperatures and pressures required for its supercritical extraction. These are the conditions necessary to re-esterify the aerogel surfaces which makes the material hydrophobic in character and stable toward variations when exposed to atmospheric moisture (E. C. Broge, U.S. Pat. No. 2,680,696). The silica aerogels made by this process have improved properties of transparency and strength, over those produced from Kistler's method. However, the silica aerogels produced by the method of the instant invention can be made in a much extended range of densities and have substantially improved physical properties.
In a commercial process, exemplified by that which is described by S. Henning and L. Svensson (Phys. Scripta 23: 698 (1982) and U.S. Pat. No. 4,402,927 by von Dardel et al.)), tetramethoxysilane (TMOS) was reacted with water in the presence of basic catalyst (NH.sub.4 OH), in a single mixing step, according to the following reactions: ##STR1## The condensation reaction immediately follows the hydrolysis in the same reaction vessel.
The microstructure of the aerogel made by this process is composed of spherical primary particles linked together to form chains which are themselves linked to form a continuous matrix of silica, surrounded by the reaction solvent, alcohol. The reaction rates of the hydrolysis and condensation steps strongly depend on the pH through the influence of the catalyst and these rates ultimately determine the microstructure of the gel (R. K. Iler, The Chemistry of Silica (Wiley Interscience, New York, 1979) and D. W. Schaefer, Science 243:1023 (1989)). Conventional silica aerogel glasses have distinguishable microstructures which are characteristic of the particular reaction process used for their formation.
Conventional silica aerogels, made by the "single-step" hydrolysis/condensation reactions given above in equations [1] and [2], have a bulk density in the range of 0.05 to 0.27 g/cm.sup.3. Stoichiometric and miscibility considerations limit monolithic aerogels attainable to a maximum density of near 0.3 g/cm.sup.3. Lower densities are achieved by dilution of the initial reactants with additional alcohol. However, the higher the dilution, the longer the time that is required for gelation to occur. Also, at some maximum dilution level, the reverse equilibrium reactions [Eq.1] will inhibit gelation, thereby setting the ultimate density limit for low density aerogels.
Aerogels are generally transparent, however, when aerogels are prepared by conventional "single-step" method, there is a loss in the clarity of those aerogels which have a density less than 0.04 g/cm.sup.3. It is believed that the loss of transparency in the low density aerogels is produced by light scattering from pores in the aerogel which have diameters greater than 100 nm.
Tewari et al., in U.S. Pat. No. 4,610,863, "Process for Forming Transparent Aerogel Insulating Arrays", described an improved process for making silica aerogels wherein alcohol that was generated in a "single-step" hydrolysis/condensation reaction of silicon alkoxide to form an "alcogel", was removed by substitution with liquid CO.sub.2 and subsequent supercritical drying of the alcogel to remove the CO.sub.2 Tewari suggested that substitution of CO.sub.2 for the alcohol solvent would allow removal of solvent at less severe conditions of temperature and pressure.
The "single-step" process described by Tewari et al., produced aerogels containing 5% silica, which would have a density of about 0.11 g/cm.sup.3. The chemistry of the "single-step" method, which Tewari et al., used to make the alcogel, limits the highest attainable density of an aerogel to be about 0.3 g/cm.sup.3 and it limits the lowest attainable density of an aerogel to about 0.02 g/cm.sup.3. These density limitations exist even with their described method of substitution and extraction of solvent Supercritical extraction of CO.sub.2 solvent following exchange with alcohol as described by Tewari et al., however, produces an aerogel with hydrophilic surfaces. Hygroscopic attraction of moisture to the surfaces of the aerogel leads to instability and eventually to collapse of the aerogel structure, if it is exposed to atmospheric moisture.
It is known in the art that the microstructure, and therefore, the properties of the dried aerogel are determined by its precursor chemistry. It is also known that the precursor chemistry is controllable through the use of catalysts to adjust the pH of the reacting solutions, through the amount of water used in the reactions, and by the reaction sequence. For example, single-step base catalysed hydrolysis/condensation of silicon alkoxide leads to a colloidal particle gel, whereas, single-step acid catalysed hydrolysis/condensation leads to polymeric gels. The importance of the reaction sequence was demonstrated by Brinker et al., (J. Non-Cryst. Sol. 48:47 (1982)) wherein a two-step process was described for making silica gels from which high density "xerogels" resulted after evaporative drying. The first step of the Brinker process involved the acid catalysed hydrolysis of silicon alkoxide using a sub-stoichiometric amount of water required to fully hydrolyse the silicon alkoxide. This first step produces a partially hydrolysed, partially condensed silica in alcohol solution, in which the presence of the alcohol limits continued condensation by affecting the reverse equilibrium reactions. The sol from this step could be characterized as consisting of clusters of polymeric silica chains. The second step involved the base catalysed completion of the hydrolysis/condensation reaction where the condensation continues until gelation occurs. The microstructure of the final gel made from this two-step process was more highly crosslinked and generally stronger than that of a single-step process gel.
Schaefer et al., (Physics and Chemistry of Porous Media II. J. R. Banavar, J. Koplik and K. W. Winkler, Eds. AIP New York (1987) pp. 63-80) described a modified two-step process in which the alcohol that is generated by the reactions of the first step was removed from the reaction by distillation, leaving a partially condensed silica intermediate. The intermediate was dissolved with another alcohol, before completing the hydrolysis/condensation with base catalyst. The alcohol in the product gel was then supercritically extracted, producing aerogel. The microstructure of the aerogel resulting from this two-step process was polymeric as in the single-step hydrolysis/condensation reactions. Neither of these references recognized the adverse effects of the alcohol solvent on the hydrolysis/condensation reactions which are due to the re-esterification of the hydrolysed species in the presence of excess alcohol. The presence of alcohol in the reactions affect both the rates and the degree of polymerization of the condensing gel, and thus affect its morphology. The presence of the alcohol also limits the gelation process to preclude the formation of very low density aerogels (i.e. densities less than 0.02 g/cm.sup.3).